Nickel electrodes are used in a variety of devices generally referred to as energy sources such as cells or batteries. Highly desirable in such electrodes is high energy density, high percent utilization of active material, high discharge and charging rates, and high recycling capacity. It is also highly desirable to achieve these electrode characteristics with a fabrication procedure which is easily adaptable to commercial manufacture and does not involve corrosive materials or excessively complex or expensive equipment.
A frequently used procedure for making nickel electrodes is known as the vacuum impregnation procedure. Here, a porous structure, usually in the form of a nickel plaque, is filled under vacuum with an aqueous solution of a nickel salt. The salt is reduced to the hydroxide by treatment with an alkaline solution, and the liquid allowed to evaporate, leaving the hydroxide. The amount of material loaded at one time is, of course, limited by the solubility of the salt in the aqueous solution. Typically, at least four impregnation cycles, requiring four or five days to complete, are required to achieve commercially acceptable electrodes by this method.
Another method is described in U.S. Pat. No. 3,214,355 issued to Ludwig Kandler on Oct. 26, 1965, involving the electrolytic deposition of nickel hydroxide directly in the pores of the electrode structure as a cathode. This is carried out in an acid electrolyte containing nickel ions and reducible ions, the redox potential of which is more positive than that of the nickel ions. During the electrolysis, the reducible ions, for example, nitrate ions, prevent the reduction of the nickel ions within the cathode structure by themselves being reduced. The resultant consumption of hydrogen ions increases the pH value of the electrolyte within the cathode structure to the extent that slow precipitation of nickel hydroxide results.
The electrolytic impregnation procedure has been further improved so as to yield high energy densities and long cycle life without significant degradation of energy density. These improvements are disclosed in U.S. Pat. No. 3,653,967 issued to R. L. Beauchamp on Apr. 4, 1972. Particularly significant is the use of high temperature (above 85 degrees C.) during the impregnation process and the inclusion of nitrite ions in the electrolytic solution to control the pH of the bath.
Particularly desirable is an impregnation procedure for nickel electrodes which involves an easily handled, simple solution, moderate temperatures without the necessity of critical control and a procedure which produces optimum use without critical control of operational parameters. Stated in more general terms, it is desirable to have an impregnation procedure suitable for commercial manufacture where the impregnation solution is not highly corrosive, where temperature need not be precisely controlled and impregnation results (energy density, cycle lift, etc.) are not critically dependent on the conditions of the impregnation procedure.